The invention is in the field of magnetic resonance, in general, and specifically pertains to improved balanced circuit performance for magnetic resonance probes.
Coupling an RF source through a transmission line to a load traditionally involves a careful matching of the load impedance to the transmission line impedance, which is again matched to the RF source. Tuning the load to the RF source frequency is a separate degree of (RF) freedom obtained through an independent adjustment. Both of these operations maximize the dissipation of the available RF power in the load. The impedance and frequency response are not the sole features which demand attention, especially as the wavelength approaches the physical dimensions of the load. It is well known in the radio communications arts that when the load is an antenna, the resulting radiation field is the ultimate design goal and the field distribution and polarization distribution will be severely affected by the electrical symmetry properties of the radiator. It has long been known that the radiation field symmetry properties are disturbed for the case where an unbalanced source drives a balanced load, or a balanced source drives an unbalanced load. A balanced load is one wherein there exists a plane of electrical symmetry such that this locus may be characterized by a static electrical potential, e.g., a virtual ground. A transmission line of choice for contemporary installations is the coaxial cable, which presents an electrically unbalanced symmetry with respect to ground. The NMR coil, as thus driven, is unbalanced unless additional circuit elements are present to restore balance. One effect of imbalance in the load is that in addition to radiation from the load, radiation occurs from this unbalanced transmission line which now supports asymmetrical RF currents, and the geometrical properties of the net radiation field will be distorted. In the time reversed case where the NMR probe is receptive to resonance de-excitation, from a sample within the coil, the resulting signal will be similarly degraded.
RF communications technology has long dealt with the problem through interposition of a balanced/unbalanced conversion device, in popular parlance, a xe2x80x9cbalunxe2x80x9d. At lower frequencies this took the form of a transformer circuit, center tapped for the balanced terminals and driven from the unbalanced terminals. At higher frequencies, e.g., hundreds of MHz, transformer coupling is not practical. The prior art has developed several approaches to balun devices and these are summarized by Stutzman and Thiele, xe2x80x9cAntenna Theory and Designxe2x80x9d, pp.183-187 (John Wiley and Sons, 1998).
A balanced NMR probe is desirable for decreasing the potential differences between portions of the coil (load) and ground and especially desirable as the wavelength associated with probe operation approach the physical dimensions of the load. A balanced probe yields a symmetric field distribution whereas the unbalanced circuit distributes the RF magnetic field asymmetrically and therefore, non-uniformly over the sample volume. Moreover, an unbalanced circuit imposes the full potential difference from maximum to ground across the sample, resulting in a larger RF electric field leading to undesirable sample heating, greater likelihood of arcing to nearby surfaces, and higher voltage tolerances upon capacitors in the circuit. Balanced NMR probes are well known, but these are commonly achieved with additional, usually lumped, circuit components. Murphy-Bosch and Koretsky, J. Mag. Res., v.54, 526-532 (1983); Probe structure often includes aspects that contribute to imbalance, e.g., internal unbalanced transmission line structures. The investigation of magnetic resonance in solid samples often utilizes transmission line components to realize high RF power required for these samples. Without additional circuit elements, a multi-resonant probe may exhibit excellent tuning and matching at its several ports while remaining essentially imbalanced. With differing RF field distributions, the concurrent resonant excitations will be spatially distributed differently, and thus the interaction of the resonant spin systems will be reduced.
Among the several objects and advantages of the invention, there is provided a balanced NMR probe achieved by surrounding (laterally and at the ground plane with an open opposite end) an unbalanced probe structure including coaxial transmission lines having outer conductor(s) with a conducting surface spaced from any outer conductor(s) and extending from the ground plane of the unbalanced probe structure by an amount xcex/4 where xcex is the wavelength associated with the resonant frequency of the probe. (The term xcex/4 is used in an approximate sense for reasons which will be discussed below.) The tuning and matching components of the probe circuit are constrained to placement within the resonant xcex/4 length of the above mentioned conducting material with the coil disposed just beyond the xcex/4 extension. This surrounding conducting material, of at least S/4 extension, functions as a balun and imposes an RF symmetry on the circuit by providing (crudely speaking) virtual parallel xcex/4 transmission lines to ground on either side of the coil. The surrounding conductor desirably extends further than xcex/4 to provide RF shielding in the usual manner.
When the coil is multiply tuned (additionally resonant at much lower frequency), the low frequency resonance (xcex much larger than dimensions of the probe), the lower frequency RF magnetic field intensity is distributed as a half sinusoid over the coil dimensions. The symmetrization of the high frequency resonant mode aligns the high frequency RF field as a half sinusoid superposed on the low frequency distribution. Thus, experiments based upon interaction of the respective resonating spin systems are optimized.